The field of the invention is that of radio communications. To be more precise, the present invention relates to a multipoint-to-point TDMA transmission system using a particular burst structure enabling the same transmission system to transmit different types of information, for example voice and data.
In a time-division multiple access (TDMA) system, each user employs a given frequency during a given time slot, the other time slots being reserved for other users. The signal transmitted by a user in the allocated time slot is referred to as a burst.
In the remainder of this description each transmitted burst is considered to include:
a guard time containing no signal at the start and/or the end of the burst, and
information symbols obtained by modulating the transmitted signal.
In the remainder of this description:
the expression xe2x80x9cunit burstxe2x80x9d refers to the signal transmitted in the shortest time slot allocated to a user, referred to as a xe2x80x9cunit time slotxe2x80x9d, and
the term xe2x80x9csuperburstxe2x80x9d refers to a burst whose length is a multiple of that of the unit burst allocated to a single user, corresponding to a plurality of consecutive unit bursts, referred to as xe2x80x9cadjacent burstsxe2x80x9d.
In the case of voice transmission, characterized by exactly the same regular bit rate for each user, each user is regularly allocated a unit time slot. The duration of the time slot is a constant of the transmission system and is matched to the characteristics of the voice transmission service in terms of bit rate, delay, or required error rate. Adaptation is required if the same transmission system must simultaneously support different types of services each having its own characteristics. Different types of information, such as voice and data, are conventionally transmitted by allocating users respective numbers of unit time slots according to their requirements.
This approach can be extended to increase bit rate, as described in U.S. Pat. No. 5,566,172, which discloses a method in which a plurality of consecutive unit time slots are grouped together and allocated to a single user who can transmit a superburst in that time slot. The grouping substitutes information symbols for redundant sequences, such as the guard times between two superburst information symbol sequences. This reduces redundancy within the superburst and optimizes the quantity of information transmitted in the superburst.
If the communications system includes an entity for amplifying the received signal before retransmitting it, for example a satellite, the inherent characteristics of the amplifier of that entity generate interference at sudden transitions in signal amplitude. Such transitions are observed each time that a guard time, in which there is no signal and which therefore has a negligible signal amplitude, intersects a flow of information having a non-negligible signal amplitude. The interference is caused by the fact that the amplifier introduces a modification to the phase of the output signal which is a function of the amplitude of the input signal. If the amplitude of the amplifier input signal varies suddenly, the phase of the amplified signal at the amplifier output varies by a large amount.
FIG. 1 shows an example illustrating the above problem.
Four transmitters 1, 2, 3 and 4 connected to respective transmit antennas 11, 21, 31 and 41 communicate with a base transceiver station 6 via communications channels and a satellite 5. The transmitters 1, 2 and 3 are of the same type and transmit voice type information. These three transmitters transmit in turn on the same carrier frequency F1 during unit time slots. In each triplet of consecutive time slots, the transmitter 1 is allocated the first time slot, the transmitter 2 is allocated the second time slot and the transmitter 3 is allocated the third time slot. The transmitter 4 transmits data type information on a carrier frequency F2 different from the carrier frequency F1. The transmitter 4 is the only one to transmit on the frequency F2 and transmits a series of consecutive superbursts occupying a time slot three times longer than a unit time slot. It therefore offers a transmission bit rate three times that of the voice type information transmitters 1, 2 and 3.
The amplifier connected directly to the receive antenna of the satellite 5 simultaneously amplifies the signal contained in the bursts which are transmitted by different transmitters on different carriers and which reach the amplifier synchronously. If all the bursts are the same length, the guard times coincide exactly and the interference has no harmful effects. However, when bursts and superbursts coexist, as in the example to which FIG. 1 relates, there are times at which the guard time of a burst received on a given frequency corresponds to one or more information symbols in a superburst received on another frequency.
FIG. 2 shows the time and frequency spreading of the information received at the satellite 5 in the case of the transmission system shown in FIG. 1. To be more precise, FIG. 2 represents a succession of bursts received in parallel at the frequencies F1 and F2 and plotted on two axes having the same time origin. The bursts B1, B2 and B3 received on the carrier frequency F1 are juxtaposed unit bursts from the transmitters 1, 2 and 3, respectively. The bursts B4 received on the carrier frequency F2 are superbursts three times the length of the unit burst from the transmitter 4.
There is a guard time at the start and at the end of each transmitted unit burst or superburst. The guard times 12, 22, 32 and 42 are start of burst guard times transmitted by the transmitters 1, 2, 3 and 4, respectively. The guard times 13, 23, 33 and 43 are end of burst guard times transmitted by the transmitters 1, 2, 3 and 4, respectively.
Each burst includes a training sequence directly after the start guard time and directly before the end guard time. These training sequences are shown in FIG. 2 but are not identified by any reference symbols.
Each of the bursts B1, B2, B3 and B4 also includes respective information symbols 14, 24, 34 and 44 corresponding to the payload information transmitted by the user to whom the burst is allocated.
The start of burst guard time 12 transmitted by the transmitter 1 is received at the same time as the start of burst guard time 42 transmitted by the transmitter 4.
The end of burst guard time 13 transmitted by the transmitter 1 and the start of burst guard time 22 transmitted by the transmitter 2 are adjacent and received at the same time as the information symbols 441 of the burst transmitted by the transmitter 4.
The end of burst guard time 23 transmitted by the transmitter 2 and the start of burst guard time 32 transmitted by the transmitter 3 are adjacent and received at the same time as the information symbols 442 of the burst transmitted by the transmitter 4.
The end of burst guard time 33 transmitted by the transmitter 3 is received at the same time as the end of burst guard time 43 transmitted by the transmitter 4.
Interference generated when the amplifier of the satellite 5 receives the guard times modifies the phase of the signal conveyed in the superburst at the corresponding locations 42, 441, 442 and 43. The locations 42 and 43 are guard times and interference here therefore has no harmful effect. The locations 441 and 442 containing information symbols are sensitive to the interference, however, which gives rise to a signal demodulation problem. Because the phase of the information symbols contained in each burst is estimated, and used to demodulate the signal, any such phase modification causes many demodulation errors in the information symbols at the locations 441 and 442.
Because the superbursts are used for data services, which generally have stricter error rate requirements than voice services, an object of the present invention is to define a superburst structure which protects the superburst information symbols from the harmful effects of amplifier non-linearities and which optimizes the quantity of information transported in the superburst.
The above objects, and others that become apparent hereinafter, are achieved by inserting filler sequences at the locations of superbursts not occupied by a guard time and likely to correspond to a location occupied by a guard time in a burst received simultaneously on a different carrier frequency.
Thus non-linearities induced by a guard time on a particular carrier frequency do not interfere with information symbols because the information symbols never coincide with guard times received at the same time on another carrier frequency. This limits the error rate caused by interference due to non-linearities of the amplifier.
In a preferred embodiment, each filler sequence inserted into a superburst is made longer than the guard time it has to cover. Its length is preferably equal to the length of the guard time to be covered plus twice the absolute synchronization uncertainty, the sequence being centered in the middle of the corresponding guard time.
The filler sequence can advantageously be a training sequence, which provides the receiver with regular training sequences in addition to solving the above problem. Introducing additional training sequences into a superburst has the advantage of improving tracking of drift in the frequency of the signal received at the receiver, which assures good demodulation quality.